1. Field of the Invention
This invention relates to an optical information reading-out apparatus such as a video disc reproduction apparatus or the like which makes use of a focusing and tracking mechanism for driving an objective lens in both focusing and tracking directions.
2. Description of the Prior Art
In an optical information reading-out apparatus such as a video disc reproduction apparatus or the like, it is required to focus an objective lens on an information track recorded on a video disc so as to cause a reading-out light flux to precisely converge onto a video disc surface and it is also required to correct tracking errors so as to cause the light flux to trace a given information track recorded on the video disc. In this case, one of problems which have heretofore been encountered in the prior art techniques consist resides in the manner of driving an objective lens with the aid of focusing and tracking signals which have been derived from the information track by a suitable means. Heretofore, it has often been proposed to arrange a galvanomirror in part of a light path of the light flux reading out the track and supply a tracking signal to the galvanomirror so as to deflect the light flux in a direction perpendicular to the track. As a result, the light flux passes in a direction which in inclined with respect to an optical axis of the objective lens or the like in response to the step of correcting the tracking error. In this case, it is necessary to eliminate not only spherical aberration but also astigmatism or the like of an optical system including the objective lens or the like for the purpose of precisely converging the light flux onto the video disc surface. As a result, the optical system becomes large in size, difficult in design, complex in construction and heavy in weight. Thus, it is difficult to provide an objective lens driving mechanism having an excellent frequency and damping characteristic. The disadvantage of such mechanism is that it is large in size and complex in construction.
It has also been proposed to correct the tracking error by displacing the light flux in parallel to the optical axis without inclining the light flux with respect to the optical axis. For this purpose, the objective lens may be constructed in such a manner that it is movable in a plane perpendicular to the optical axis. In this case, if the movement of focusing the objective, that is, the movement of the objective lens in the direction of the optical axis is taken into consideration, the objective lens must be so constructed that it is movable not only in a plane perpendicular to the optical axis but also in the optical axis direction.
If a mechanism for driving and focusing the objective lens in the optical axis direction is enclosed in a lens holding body, the mass of a movable member for correcting the tracking error becomes large, and as a result, the mechanism as a whole becomes too large in size to be able to effect an excellent control. That is, the focusing movement of the objective lens is required to be larger in amplitude than the movement of the objective lens for correcting the tracking error by the order of one figure. As a result, the driving mechanism for focusing the objective lens becomes large in size by all means. It is very difficult, therefore, to construct such driving mechanism in a manner that it can not only focus the objective lens but also correct the tracking error thereof.